TOKYO, Jan. 13 (Yonhap) -- North Korea test-fired three short-range ballistic missiles into the East Sea earlier this week, apparently as part of efforts to improve its missile technology, a Japanese newspaper said Friday.

North Korea launched what appeared to be KN-02 ground-to-ground missiles on Wednesday morning, according to multiple Japanese government sources quoted by the Sankei Shimbun.

The communist country also test-fired two short-range missiles on Dec. 19, shortly after it announced the death of its longtime leader Kim Jong-il. South Korean officials said they did not see the launch as anything more than test-firing.

The South's military believes North Korea is developing KN-06 missiles, which are an improvement over the KN-02, and has been test-firing them since last year to check their performance before deploying them.